It is well known that rotary wing aircraft are capable of flight in a vertical direction as well as forwardly or backwardly in horizontal flight, and may further remain stationary while aloft. Examples of rotary wing aircraft correspondingly include helicopters and tilt-rotor aircraft, among others. In all of these flight vehicles, thrust is generated by one or more rotor assemblies having aerodynamically-shaped blades that are constantly in rotational motion relative to the flight vehicle. The generated thrust may be used to lift and/or propel the rotorcraft in a lateral direction. Due to pressure differences that exist between an upper surface of the blade and a lower surface while the blade is in motion, vortices are generated at the tips of the blades that produce various undesirable effects.
One notable effect is an increase in drag on the blades of the one or more rotor assemblies, particularly while the rotary wing aircraft is stationary, or hovering. Consequently, an engine that drives the one or more rotor assemblies must develop additional torque to overcome the pressure drag generated by the tip vortices, resulting in higher fuel consumption for the rotary wing aircraft. Further, since the tips of the blades generally operate at relatively high subsonic local Mach numbers (typically between M=0.6 to 0.95), an increase in compressibility drag may occur that is due to shock wave drag or shock-induced flow separation, particularly on an advancing blade tip when the vehicle is in forward motion.
Another undesirable effect of the tip vortices is the generation of noise and impulsive changes in aerodynamic loads on the blades. In particular, the interaction of vortices shed from a blade tip and a following blade, and/or the interaction of the tip vortices with the fuselage or empennage structures of the rotary wing aircraft, result in a phenomenon known as blade-vortex interaction (BVI), which may be particularly severe when the blades are relatively heavily loaded, such as during a low-speed descent to landing, or during the execution of certain flight maneuvers. Since BVI generates elevated vibration levels in various components comprising the rotary wing aircraft, the useful life of such components is typically shortened. Additionally, occupants in the rotary wing aircraft may experience the unpleasant effects of the elevated vibration levels generated by BVI.
Various prior art devices have been employed in an attempt to address the foregoing undesirable effects. For example, rotor blade tips have been configured with a sweep back angle, or with a localized span-wise twist. Still other prior art attempts have utilized improved airfoil cross-sections for the rotor blade, or have employed tips having a negative dihedral angle. The foregoing devices are generally effective under a specific flight condition, and are relatively less effective under other different flight conditions.
Therefore, a need exists for improved apparatus and methods that mitigate the undesirable effects of tip vortices more effectively and over a greater range of flight conditions than achieved by the prior art.